Several tech companies argue heavily for introducing DC energy grids even in diesel electric propulsion solutions.

One of their arguments to do so - are caused by the idea to save extra fuel by controlling the speed of the diesel generators especially in low-load conditions. But DC grid is an old technology that was abandoned before it was properly developed in favor of the much more efficient AC technology. AC has several well-known advantages with a huge range of proven components available all over the world.

STADT has evaluated AC - towards DC grid from our Lean Propulsion® perspective. We concluded with our STADT VARIGRID™ - solution, which achieve the extra fuel-reductions by combining the already fuel-efficient STADT Lean Drive with variation in AC-grid frequency. This performs variable speed for the diesel generators, solved by simple, efficient, and proven AC technology that is readily available and scalable for any configuration and power need. 

In huge contradiction to the dominating AC grids that are used on majority of ships - the DC grids need to step 50 years back in time – and re-introduce Melting-FUSES (ref pictures) as the overload protecting device in their main power distribution switchboards. Melting fuses as we see in a brand-new ship with DC grid under delivery now in Asia - is for us at STADT a shocking experience. - How can the Class and ship owners dear to approve such an old fashion and sensitive solution?

The special fuses that we also see here (picture), are bolted to the main DC copper busbars. They are not resettable, not possible to isolate, not adjustable, and not easy to buy in the next harbor. AND: when they fail, replacement must be done by the electrician after having shut down the entire energy system on this ship. All these factors reduce the overall availability and Reliability of the propulsion.

If Bus-tie breakers are introduced in DC grid, newly developed Transistorised “FUSES” are now in development.

Complex items that need Water cooling due to its extreme power losses of 20 kW for a 3000 Amp rating as shown here (FX).

For comparison, the AC equivalent, using an air-cooled ACB, exhibits remarkably low fractional losses of only 0.6 kW. - No further comments needed hopefully.

The FUSING and Protection technology used in the DC grids, is just one of many issues that pose serious concerns in a DC solution.

Never forget that the vast majority of electricity generation worldwide relies on AC generators, and the same with propulsion AC motors. A 1000 V DC can in fact hardly be used directly to ANY consumer onboard a ship.  So, DC in between makes no sense in fact.  DC grid is not a silver bullet – by far.

CONCLUSION:

STAY AWAY FROM DC GRID.  SPECIFY AC GRID WITH FIXED OR FLOATING FREQUENCE – STADT VARIGRID™.